The invention relates to a method for the excitation of a ply in which a gas is subjected to an electric field by means of at least two separate electrode systems, which are power supplied from separate generators of the same frequency, the voltage variations of the generators being shifted in phase relative to each other.
It is often advantageous to use a low-frequency alternating voltage instead of DC voltage for generating a plasma in order to prevent the formation of constant sparks between the electrodes. In this connection low-frequency signifies frequencies above 2 Hz, but below 10 Hz. One of the advantages of using low-frequency alternating voltage is that an impedance matching between the generator and the plasma is not required. Another advantage of using low-frequency alternating voltage is that reactive losses in power supply lines, feed-throughs and the like can be ignored, which simplifies the equipment design significantly.
The known systems use an electrode or an electrode system supplied by one alternating voltage. However this configuration only render few possibilities for adjusting the intensity and homogeneity of the plasma to the particular need
Furthermore it is known from EP 0831679 to discharge the voltages for generating the plasma from one phase by means of a power portion, the individual portions of the power being substantially equally shifted in phase relative to each other.
The object of the invention is to show how to provide AC electrode systems which are able to meet several different requirements of intensity and homogeneity in the generated plasma.
A method of the above type is according to the invention characterised by choosing the phases and/or the amplitudes in an asymmetrical way so as to obtain a kind of a periodical rest period in which there is no significant potential difference between the phases, voltage zero never occurring simultaneously in two of the electrode systems.
As a result a pulsating plasma is obtained, as the plasma is powered by the potency difference between the phases. In the part of the period in which no particular potential difference exist between the phases, an added substance may interact with its own functionality.
In this context generator signifies a voltage/power source, in which the phase of the alternating voltage on the output is substantially insensitive to the load caused by the plasma. A generator may be a transformer connected directly to one of the three phases of the mains and zero or between two of the three phases of the mains. A generator may also be formed of another voltage source rendering an oscillating signal as an output signal, the frequency of which being determined by a local control circuit.
By using two electrode systems a plasma with relatively low intensity is obtained in relation to a plasma with many electrode systems that are all supplied with the same voltage.
Moreover according to the invention the electrodes may be placed along a circle, the plasma being generated in the center of the circle. As a result the plasma is generated in an inhomogeneous zone adjacent the electrodes, while the central part of the chamber is filled with homogenous xe2x80x9cdiffusionxe2x80x9d plasma. The conditions in the plasma in the homogenous central part are such that reactions, which normally would not be produced in conventional plasma equipment, can be obtained, as the molecules are only broken into smaller fragments in this area. The plasma is thus xe2x80x9cgentlexe2x80x9d towards an added substance, such as an added monomer.
In another configuration the electrodes are placed along a cylindrical body which in turn is encased in a tube, the plasma being generated between the electrodes and the interior of the tube. During the generation of the plasma the tube is thus able to rotate slowly.
In a particularly advantageous embodiment the phase shift between two phase may be "PHgr", where
0.5 less than "PHgr"+1xc2x7xcfx80 less than 2.5
where 1 is a positive integer. By selecting an asymmetrical phase shift, the intensity of the plasma may be varied during the oscillation period, eg by displacing the two phases by 30xc2x0 such that a pulsating plasma is generated, the plasma being generated by means of the potential difference between the phases. As a result an added monomer is able to interact with its own functionality, ie to obtain a form of equilibrium in the part of the period when there is no significant potential difference between the phases.
At higher voltage amplitudes of one of the phases it is possible to perform tasks, in which for instance a special geometry of the blank implies special requirements. As an example it is often necessary in an internal plasma processing to place an electrode inside the tube or hose and to impress a higher voltage on this electrode than on the electrodes generating the rest of the plasma.
Optionally according to the invention three or more discrete electrode systems are used, which are power supplied from separate generators, at least two of the said generators being of the same frequency and used to generate an AC plasma, the voltage variations of the generators being shifted in phase relative to each other, and at least one of the electrode systems comprising at least two electrodes. The third phase may thus for instance be used for depositing a metal coating by a cathode sputtering process. Optionally this phase can also be for cleaning the surface of a blank.
The invention further relates to a use of the method according to the invention for excitating a fluorescent material.